In most threads I mention salt (NaCl) is simply salt when treating for fresh water diseases like ick.
Sea water mix is "not" just salt.
It is a combination of salt(s) minerals, buffers and other components that are needed by sea water fish and inverts.
Without these extra component of sea water, many quickly die, and invert shells fall apart.
If you want a salt water tank, and inverts, you must use a sea water mixtures, like Instant Ocean, or Reef Crystals etc, not just salt.